Field of the Invention
The invention relates generally to seals for spectroscopic measuring devices and, more particularly, to an optical transmission cell with minimized spurious absorption.
Description of the Related Art
Spectroscopic measuring devices are often used to analyze gasses. The purpose of this invention is to solve a problem that has arisen in the production of transmission cells for the analysis of gasses using near infrared spectroscopy. The problem will be discussed with reference to FIG. 1 which shows a transmission cell 110, transmission cell system 110, that is designed for analyzing a gas via operative connection to a spectrometer (not shown). As shown, the preferred transmission cell 110 consists of three major subassemblies: a central section or body 20 formed from a standard Swagelok® cross fitting, and first and second optical probes 40, 50 which are directed toward each other across the central “cross” of the cross fitting 20.
The transmission cell 110 functions as follows: Near-IR radiation (which we will refer to as “NIR light” or just “light”) enters the system 110 by means of a transmitting fiber-optic cable (not shown, but alluded to with the inbound arrow labeled “NIR Light” on the left of FIG. 1). This fiber is terminated into the first optical probe 40 at a fiber-optic connector 41. The light diverging from the end of the transmitting fiber is collected by a lens 42 so as to form a nominally collimated beam that is directed through an interior of the first probe 40 (region 1), through a measurement space in the cross fitting or cell body 20 (region 2), and then through an interior of the second probe 50 (region 3) where it collected by a second lens 52 and focused on a receiving optical fiber (not shown, but alluded to with the outbound arrow labeled “NIR Light” on the right side of FIG. 1). The receiving fiber would be located in connector 51 in the same manner that the transmitting fiber would be located in connector 41.
A total of four optical windows 44, 45, 54, 55 are included in the preferred transmission cell 110. The number is four because each probe 40, 50 has two windows—a primary one and a secondary one. In more detail, the first probe 40 has a primary window 44, and a secondary window 45 and the second probe 50 has a primary window 54 and a secondary window 55.
As further shown in FIG. 1, the two primary windows, 44 and 54 face one another from opposite sides of the nominal measurement region (region 2). The primary windows 44, 54 are preferably sealed into the tips of the first and second probe bodies 40, 50 using Axiom's patented welded sealing technique utilizing metal “C” rings 43 and 53 coated with a compliant material such as PTFE or Gold. These seals are designed to withstand the high pressures that may be present in the measurement region (region 2) and are explained in more detail in U.S. Pat. No. 6,587,192 hereby incorporated by reference as if fully set forth herein.
The secondary windows 45 and 55 are located near the outer ends of the first and second probes 40, 50. The purpose of these secondary windows 45, 55 is to isolate the probe interiors (regions 1 and 3) from contaminants that may be present in the ambient air. The space between the primary and secondary windows, i.e. between windows 44 and 45 and between windows 54 and 55, forms the interior of a given probe (regions 1 and 3). Each region has a diameter and a length which we will specify as Dp and Lp, respectively.
As further shown in FIG. 1, the probes 40, 50 are characterized by interiors, bores, or unfilled volumes 47, 57 in regions 1 and 3. It would be desirable to minimize Lp as much as possible to minimize the effects of any undesired gases in the probe body 40 or 50. However, a practical minimum value is placed on Lp by the fact that the probes 40, 50 must interconnect, e.g. have some length to mate to the mechanical dimensions of the Swagelok cross 20 with threaded fasteners (not separately numbered). The effects of undesired gasses within the interiors, bores, or unfilled volumes 47, 57 of the probes 40, 50 could also be minimized by continuously pumping on the probes 40, 50 or purging them. However, this is not practical in many situations such as when the systems 110 are located in the field, such as at natural gas wells. At first blush, the only practical approach in such case is to first evacuate the interior of the probes 40, 50 and then back fill them with an inert gas such as nitrogen.
Even though the seals 43, 53 have extremely low leak rates, the vapor in the cell body 20 is often at very high pressure relative to the pressures in the probe bodies 40, 50, such that over time small amounts of gas will inherently leak into the probe bodies 40, 50. This will create measurement problems because the measurements are very sensitive to small changes in the NIR spectrum.
There remains a need, therefore, for a spectroscopic measuring devices such as a transmission cell that resolves such problems, i.e. that minimizes spurious absorption due to such leakage.